Regrinding badly scratched front lens of M plan 20x ELWD?

[viktor j nilsson]

Some years ago, I took a chance on a Nikon M Plan 20x ELWD on eBay for a (suspiciously) low price. Sadly, I did not strike gold. The front lens is badly scratched and etched, sand blasted almost:




I have no idea what caused this.
It's a bit similar to ChrisR's objective here: http://photomacrography.net/forum/viewtopic.php?t=9612
...but less evenly spread over the front lens, and with a lot more scratches.

Image quality when stacking is quite poor. Lots of flares. The worst thing is that the defects are not evenly spread over the frame, so it is very hard to fix in post processing. I can upload some example later.

Visually, in a microscope, the picture looks ok-ish. But my BD plan 40x ELWD is obviously far, far superior in terms of sharpness and contrast.


I don't think that I'll ever use this objective as it is.

This got me thinking: could this be a candidate for a regrinding of the front lens using cerium oxide? As suggested by g4lab here: http://photomacrography.net/forum/viewt ... 7e9f6dd9f7

This would of course remove the lens coating and cause at least some changes in the shape and thickness of the front lens. I have no idea if this would make the lens better or worse. But since I have no use for the objective as it is, this could be an interesting case study.

Whaddayasay, should I do it?

[enricosavazzi]

Grinding the lens by hand against a cloth or similar is unavoidably going to introduce serious wavefront errors. In one word, the lens becomes unusable.

The way lenses are ground today is by machine, rubbing two flat surfaces against each other sideways by axially loading and moving one of them while one slowly rotates against the other. The sideways rubbing causes one of the surfaces to became concave, the other convex. Rotation forces the two surfaces to assume a radially uniform profile by evening out irregularities. The net result is two spheric surfaces. Then the two surfaces must be polished, in a similar way but with much finer abrasives. Some amateur astronomers used to make their own telescope mirrors manually, in a similar way. I don't know whether they still do, but scaling down the process to a lens only millimeters in diameter makes it a lot more difficult to replicate by hand.

I would have no idea how to grind only one lens that already has a spheric surface, instead of starting with a matching pair of flats.

[viktor j nilsson]

Thanks for tour input!

I agree, grinding the lens against a cloth would ruin it.

However, I still believe that it might be worth polishing it.

My current plan, based on reading a lot about mirror polishing in astronomy, is to create a polishing lap out of beeswax. I would build a "dam" around the front lens and pour hot wax into it to create a perfect negative of the lens shape. I would then use it to polish the lens using cerium oxide.

Btw, I found this page about how they hand polish Nikon microscope lenses:
https://www.nikoninstruments.com/en_EU/ ... icroscopes

I'm thinking of doing something a bit similar: rotate the polishing lap (facing upwards) and carefully hold the front lens against it.

Ideally, you would remove the front lens when doing this. I seems pretty easy to do, but I'm worried I would mess up the collimation.

[houstontx]

thanks for the info i've had the same problem recently...might have to try your method...
http://www.photomacrography.net/forum/v ... hp?t=36308

[viktor j nilsson]

thanks for the info i've had the same problem recently...might have to try your method...
http://www.photomacrography.net/forum/v ... hp?t=36308

I did see your post when I was searching for information about ways to deal with lens damages. Have you tried taking pictures with your objective? Is it unusable? Your front lens is much smaller than the fairly large and relatively flat front lens of my 20x ELWD. That might make it harder.

I'm surprised that so few seems to have tried to polishing microscope lenses themselves. It might be because veryone that has tried it has failed.

I'll keep you updated on how this goes if I decide to do it. I think I will. If nothing else, it might help others decide what to do with their eBay mistakes.

Btw, I also found this book which has a ton of useful information:
Prism and Lens Making, Second Edition: A Textbook for Optical Glassworkers
https://books.google.se/books?id=TQG8K7vjwNkC

On p90, he writes that a lap made with mix of 3.5 parts beeswax and 2 parts resin is "exceptionally good for avoiding scratches". I might try that. Resin can easily be found in the form of violin resin (colophony).

My main worries are currently:

1) Should I disassamble the objective? Cerium oxide is generally used mixed with water. So there is a great risk that water and CeOx could get into the objective and destroy it. On the other hand, I really don't want to get it out of adjustment. By holding the objective against the polishing lap with the front lens facing downwards, I'm hoping that gravity might help keep the water out of the objective.

Alternatively, I've read that Cerium oxide can be used dry by sprinkling it onto a good quality paper attached to the polishing lap. But that seems really hard to do when the surface isn't flat.

I also read somewhere that the polishing agent (here: cerium oxide) could be mixed directly into the wax when making the polishing lap. I wonder if this could be possible in this case.

2) Should I polish using motor tools or by hand? My first plan was to mount the polishing lap on a drill or similar. However, maybe I would have better control if I just moved the objective over the polishing lap by hand. But that might take forever... Decisions, decisions.

[Troels]

There are still amateur astronomers who grind and polish their own mirrors.
Some use a motorized plant for the rough grinding.

When it comes to the final figuring it is definitely handwork. Just of a few careful strokes will make the difference beween a good and a perfect mirror - or between a near perfect and a good mirror.

Your problem is: You can't remove a thin layer of glass with scratches without changing the figuration of the lens. And contrary to the mirror-polishers in the astronomy club you don't have any tools to mesure how far you are from the perfect figuration, and where you have to place the next stroke to correct the errors.

[soldevilla]

I made many astronomical mirrors years ago.
A basic rule is that you can not eliminate large defects with small solutions. If your lens has stripes, you must apply abrasive until they disappear, and once you get a uniform matte surface (you need a magnifying glass to see this) then you must polish until the lens becomes transparent again. Polishing does not change the figure excessively if you have a suitable tool of very hard tar, but working with carborundum to eliminate the scratches will completely destroy your optic's figure. In my experience, you have better possibilities carving a new lens than trying to recover the lens

[viktor j nilsson]

Thanks for the input.

I don't plan to remove too much material here. The vast majority of the surface defects are very shallow. I don't care if there are some scratches left after I've polished the lens, since a few chips and scratches rarely have a major impact on IQ. My hope is that smoothing out the "sand blasted" regions will improve the image quality more than the inevitable changes in lens shape will deteriorate it.

As Soldevilla says: "Polishing does not change the figure excessively if you have a suitable tool of very hard tar". That's exactly what I want to do.

If I wanted to get rid of every single defect, I would probably have to begin with a coarser abrasive. I don't want to do that. So the plan is to only use 2-4 micron Cerium Oxide on a wax lap. I might pick up some pitch and make the lap out of pitch/beeswax. But I do like the idea of using beeswax and rosin, even if it is a bit softer.

[viktor j nilsson]

And troels, I believe you are right that this should be made by hand, not using motor tools.

[GaryB]

Is the glass itself scratched or just a badly worn/scratched coating?
That pic looks very similar to a cheap 'sold as-is' Wild fluotar 40 .75 I bought. Once I polished off the lens coating it looked great and worked much better. Even if it's the glass, just removing any coating should help a lot.

[viktor j nilsson]

GaryB, that's extremely encouraging!
Please tell me, how did you go about polishing off the coating?

I'll put it under the stereo microscope again this afternoon and I'll try to see and feel how deep the marks are. The many small flecks I believe are very shallow. However, I guess that lens coatings are so thin they might still go a little bit into the glass.

[GaryB]

The lens was sold as junk so I didn't have any fear... I put some very fine polishing paste on a soft micro buffing wheel and hit it with a low speed rotary tool
Because the coating (much thicker than you'd imagine on that Wild) was so badly scratched and chipped, it came off quickly and easily, leaving pristine glass beneath. The glass is much harder than the coatings adhesion, so no apparent damage was done.

I also believed that the glass was scratched before I buffed it, but it wasn't!
Good luck!

[viktor j nilsson]

GaryB,
I've looked closely at the damage at 50x under my Wild M5. There is one scratch that goes a little deeper than the rest. But even that one is not deep, I can just barely see (or sense) that it forms a tiny "valley" when viewed at 50x. The rest of the damage is made up of thousands and thousands of minute spots. I am starting to hope that polishing off the coating will be enough.

[ChrisR]

That lens of mine almost looks like some vapours have attacked it.
Are there any chemicals known to cause that? Methylene chloride? Maybe a specimen had been etched with hydrogen fluoride and they forgot to wash it all off ?

[siliconGary]

That lens of mine almost looks like some vapours have attacked it.
Are there any chemicals known to cause that? Methylene chloride? Maybe a specimen had been etched with hydrogen fluoride and they forgot to wash it all off ?

A bit off topic but back in the 80's I worked in a chip wafer fab in the UK. The microscopes in the etch area (Where there was lots of HF etching of silicon oxide) were all a bit bad. You quickly knew to look at something interesting you took it next door into the lithography area (where only solvents were used)